Conventionally, a webbed spiral bevel gear is employed for a driving force transmitting member such as a differential gear unit for an automobile, which webbed spiral bevel gear exhibits a high gear strength and causes a small transmitting noise when operating at a high-speed rotational level. JP2001-205385A discloses a method for manufacturing a webbed bevel gear by cold forging.
FIGS. 4 and 5 illustrate a conventional webbed spiral bevel gear 21 formed by cold forging. The webbed spiral bevel gear 21 includes a gear basis 22 and gear teeth 25 each including a tooth top land 23 and a tooth bottom land 34. In each gear tooth 25, the tooth top land 23 and the tooth bottom land 34 are both formed integrally at a peripheral surface of the gear basis 22 and extend slantingly and curvedly relative to a center axis of the gear basis 22. The webbed spiral bevel gear 21 further includes a web portion 26 formed with a radially outer end portion of each gear tooth 25 to be integral therewith and closing the tooth bottom land 34 at the radially outer end portion of each gear tooth 25.
However, as illustrated in FIG. 5 (enlarged diagram of portion “B” of FIG. 4), in the conventional webbed spiral bevel gear 21 formed by cold forging, a cross-section c-c is taken along a direction perpendicular with the center axis 34 of the gear basis 22 from the gear tooth 25 to the web portion 26. As illustrated by the cross-section c-c, a profile of the radially outer end portion of each gear tooth 25, which is continuous with the web portion 26, rapidly expands at the web portion 26 in a direction from the radially outer end of the gear tooth 25 to the web portion 26. Accordingly, at the time when the gear is formed, smooth flow of gear material in a mold (not illustrated) is difficult. Therefore, a large forming load is required for processing. Increase of the forming load causes a shorter mold lifetime and causes an increase in processing cost.
Further, when the forming load increases, forcible tensile stress and compressive stress are applied to the gear material. Accordingly, reproduction accuracy for the material worsens, which causes a decrease of processing precision. The decrease of the processing precision causes necessity of post-processing (cutting, or the like) or a gear noise (transmitting noise), or the like.
Further, in a structure of the webbed spiral bevel gear 21, an acute angle side and an obtuse angle side are provided between each gear tooth 25 and the web portion 26. When the webbed spiral bevel gear 21 is utilized as a transmission component, a concentration of stress between the gear tooth 25 and the web portion 26 at the acute angle side may occur, causing a decrease of gear strength.
A need thus exists for a webbed spiral bevel gear, which is not susceptible to the drawback mentioned above.